The present invention relates generally to deposition of semiconductor layers and specifically to a process for producing an epitaxial layer of gallium nitride (GaN) by the method of hydride vapor-phase epitaxy (HVPE), as well as to the epitaxial layers which can be obtained by said process.
Substrates including epitaxial GaN layers are used in optoelectronics. In particular, intense blue to ultraviolet light emitting diodes (LED) are made from these GaN layers.
HVPE is a widely used technique for producing GaN layers. Two of its main characteristics are as follows: versatility of the growth rate (from 1 xcexcm/h to hundreds of xcexcm/h) and a vapor phase being close to the thermodynamic equilibrium to obtain localized epitaxies.
In the HVPE process, the following gases are introduced: the carrier gas (nitrogen, helium, argon, hydrogen or a mixture thereof), GaCl as a source of gallium, which can be produced in situ from HCl and Ga in the liquid form, and NH3 as a source of nitrogen.
In the method of hydride vapor-phase epitaxy that is often used as technique of quick growth of GaN, the reactors have hot walls to assure the stability of transportation agents of elements of Group III, in the form of GaCl, obtained by reacting gaseous HCl with liquid gallium inside the reactor.
The transportation agents of elements of Group V are hydrides (i.e., ammonia) which are brought into the reactor by a separate gas line. The vapor phase (GaCl+NH3) is transported towards the substrate zone of the reactor, where the substrate is located, by the carrier gas (nitrogen, helium, argon, hydrogen or a mixture thereof).
The pressure inside the reactor is one atmosphere or less. The reactor is a quartz tube of high purity. The gasses have electronic quality, that is to say of a purity superior to one ppm. In carrier gasses, such as nitrogen or hydrogen, each impurity is in a concentration less than one ppm. For the other gasses, the sum of concentrations of each impurity is less than one ppm. The metallic sources present a purity 7N, that is to say 99.99999% purity rates. The reactor heating system in which the quartz tube is located is usually an oven containing several distinct zones:
a source zone in a portion of the quartz tube where the container of liquid Group III metal source is located (i.e., the liquid gallium source),
a substrate zone where the substrate is placed during the growth,
and optionally, a central or mixing zone between the source and substrate zones, where the homogenization of gasses is performed.
The GaN layers are generally grown on the substrate at temperatures in the substrate zone of up to 1000xc2x0 C.
The main problem concerning the growth of GaN by HVPE is the presence of a parasitic deposition, also called parasitic nucleation, in front and above the substrate on the walls of the quartz tube of the reactor. This parasitic nucleation leads to an undesired modification of the composition of the vapor phase above the substrate by the consumption of GaCl and NH3 on one hand, and the production of excess HCl and H2 on the other hand. This modification of the vapor phase is hardly quantifiable. The control and the reproducibility of growth results are therefore altered. Besides, it is difficult to realize selective epitaxies in the presence of parasitic nucleation. Other inconveniences of this parasitic nucleation are the deterioration and weakening of the walls of the quartz tube reactor, and a strong unintentional doping the epitaxial layer.
The carrier gas can be hydrogen, an inert gas chosen from nitrogen, helium, argon or a mixture thereof. In the case where the carrier gas is an inert gas, the parasitic nucleation is very important because it affects the time for GaCl3 gas to achieve the equilibrium in the vapor phase. The supersaturation is then very important at the exit of the gas sources tubes, which provide NH3 and HCl for the GaCl generation. In the case where the carrier gas is hydrogen, it is easier to reduce this parasitic deposition, but to the detriment of the growth rate of the epitaxial layer.
Consequently, it is desirable to obtain a higher GaN growth rate with the lowest possible parasitic nucleation.
A first preferred embodiment of the present invention provides a method of producing an epitaxial layer of gallium nitride on a substrate by hydride vapor-phase epitaxy (HVPE), comprising providing gallium nitride source gases to the substrate, providing an additional flow of HCl gas to the substrate, and maintaining classical negative relative supersaturation conditions during deposition of the gallium nitride layer.
A second preferred embodiment of the present invention provides an epitaxial layer of gallium nitride on a substrate produced by hydride vapor-phase epitaxy (HVPE), comprising providing gallium nitride source gasses to the substrate, providing an additional flow of HCl gas to the substrate, and maintaining classical negative relative supersaturation conditions during deposition of the gallium nitride layer.